1. Field of the Invention
The present invention relates to the art of electrochemical cells, and more particularly, to an improved method of connecting a current collector to a terminal pin. The present invention is of a laser welding method by which a braze-like weld joint is formed between the terminal pin and the current collector.
2. Prior Art
The recent rapid development in small-sized electronic devices having various shape and size requirements requires comparably small-sized electrochemical cells of different designs that can be easily manufactured and used in these electronic devices. Preferably, the electrochemical cell has a high energy density, and one commonly used cell configuration is a prismatic, case-negative cell design having an intermediate cathode flanked by opposed anode components in contact with the casing and in electrical association with the cathode.
The diverse variety of materials used in the construction of electrochemical cells increases the difficulty of assembling and manufacturing such small intricate devices. It is desirable to build such electrochemical cells with simplified procedures that create an electrochemical cell with a durable and robust construction. Such electrochemical cells require joining various internal components, composed of differing materials, with a strong durable bond. One of these critical connections is that of the terminal pin to the current collector. The terminal pin connects the electrochemical cell's internal current collector to a load such as an implantable medical device.
However, because of the diverse materials with their respective distinct material properties, it is sometimes difficult to join and bond these components together. Prior art bonding techniques, such as traditional laser and resistance welding practices, are not always ideal in joining components such as terminal pin and current collector materials.
Specifically with respect to electrochemical cells, joining the terminal pin, typically composed of molybdenum, to that of the current collector, typically composed of aluminum or titanium, has been historically problematic. The diverse material properties, particularly the difference in melting temperature between molybdenum and aluminum or titanium, create problems in joining these different materials directly together. In the case of traditional laser welding, two work pieces, which are desired to be joined, are first positioned in direct contact with each other. An energized laser beam is then directed at the work piece interface, thus fusing them together. In doing so, a heat-affected zone is typically created in which an inter-metallic bond between the fused materials is formed.
The formation of such an intermetallic bond, within the heat-affected zone, may create an undesirable brittle bond. Furthermore, it may not be possible, given the distinctive compositions of the work pieces, to join such materials using traditional laser or resistance welding techniques. For example, materials exhibiting a wide difference in melting temperatures, such as molybdenum, having a melting temperature of about 2,617° C. and aluminum, having a melting temperature of about 660° C. may be difficult to join together. The heat required to melt molybdenum may significantly damage the lower melting temperature aluminum.
Therefore, to address this problem, a coupling sleeve 53 (FIGS. 2 and 3) generally comprising aluminum, was developed to facilitate bonding of the terminal pin to the current collector. In this prior method, the sleeve 53 is positioned over the molybdenum terminal pin. After the sleeve 53 is properly positioned over the terminal pin, a laser beam is typically focused through a side sleeve opening 55 directly applying the beam onto the exterior surface of the terminal pin at a perpendicular orientation with respect to the exterior surface of the terminal pin.
The intensive heat from the laser beam burns a cavity within the side of the terminal pin. This cavity is then typically filled with a second material, which creates a metallurgical bond between the coupling sleeve and the pin. Such a cavity is generally not desirable. Formation of the cavity within the side of the pin decreases its cross sectional area, and may decrease the mechanical strength of the pin. Furthermore, the addition of the second material within the cavity may create a brittle bond. In addition, this prior art laser welding technique requires exacting precision in bonding the materials together, which adds manufacturing complexity. Furthermore, such intermediate materials and processes can create brittle bonds that may not be sufficiently robust.
The present invention provides an improved means of joining dissimilar materials. More specifically, the present welding method enables an improved joining of different materials that are typically utilized in the manufacture of electrochemical cells. The present invention provides a laser welding method that utilizes heat generated from a laser beam to create a braze-like joint between work pieces. Thus, the need to burn a cavity within the external surface of a work piece is eliminated. Furthermore, the present invention eliminates the creation of an intermetallic bond within the weld connection. The laser welding method of the present invention reduces cost, complexity and creates a more robust connection. Lastly, the laser braze welding process of the present invention is fast, simple, easy to control and effective.